1. Field of the Invention
The present invention relates a novel SiCxe2x80x94C/C composite material usable as jigs for molten metal requiring oxidation resistance at high temperatures, such as molten metal carrying pumps used in molten metal or molten metal pumps for removing dross, grinding members requiring oxidation resistance at high temperatures, sliding members such as rolling bearings and plain bearings used in apparatuses for making semiconductors, precision instruments, cars and aircraft parts, and braking members used as friction materials for brake disks fitted so as to gear with speed control devices used for stopping or controlling of speed of mass-transportation means such as large cars, ultra-high speed trains, and aircraft. The invention further relates to a method for producing the composite material.
2. Related Art Statement
Aluminum-zinc alloys, SUS and other various alloys are used as jigs for molten metal, but since they are used at high temperatures, the life thereof is short, namely, about one week. Therefore, they must be frequently changed, and the changing operation at high temperature is very difficult. Thus, demanded are those materials which do not give substances contaminating the metal in the molten metal and are high in abrasion resistance, oxidation resistance and endurance, and can be used for a long period of time at high temperatures.
For example, plating of steel sheets for cars is carried out by dipping the steel sheets to be plated in metals such as zinc and aluminum molten by heating (molten metal). However, when the materials to be plated are repeatedly dipped, impurities in the molten metal grow in the form of particles to cause formation of solid suspended materials (dross) in the molten metal. If the plating is continued as it is, plating thickness becomes uneven or appearance of the plated articles becomes poor.
Therefore, in metal plating step, the plating operation is carried out with removing dross by a molten metal pump. FIG. 9 shows an example of the molten metal pump. In FIG. 9, steel sheet 11 which is a material to be plated is plated by dipping it in molten metal 13 by means of pulley 12 in a hydrogen atmosphere. Molten metal pump 14 is generally provided with dross storage portion 15 and dross passage 16 having openings at both ends. One end opening of dross passage 16 communicates with molten metal 13 outside the dross storage portion 15 and another end opening of dross passage 16 communicates with molten metal 13 inside the dross storage portion 15. Furthermore, the dross passage 16 has impeller 18 for bringing about a liquid flow from one end side to another end side fitted to revolving shaft 17 in another side of the dross passage. In the molten metal pump 14 shown in FIG. 9, the inner space of inner container 20 forms the dross storage portion 15 and the space between the inner wall of outer container 21 and outer wall of the inner container 20 forms the dross passage 16.
Sliding materials such as rolling bearings and sliding bearings are widely used in various fields such as semiconductors, ceramics, electronic parts and manufacture of vehicles as constitutive members of apparatuses for making semiconductors, precision instruments, cars and aircraft parts. Especially, at present, with the rapid progress of technical innovation, sliding materials used for sliding bearings, sliders, bearing holders, etc. in the fields of space development such as shuttle spacecraft and spaceplanes, and fields of energy such as nuclear energy, solar energy and hydrogen energy, are used at high temperatures of higher than 400xc2x0 C., at which oil cannot be used as a lubricant owing to burning or carbonization, or at low temperatures at which oil freezes. Therefore, sliding materials per se must have a kinetic coefficient of friction as small as possible and must hardly be worn. Furthermore, naturally, these sliding materials are required to have high strength and high reliability (tenacity and impact resistance) at moderate to high temperatures (200-2000xc2x0 C.), and environmental resistance (corrosion resistance, oxidation resistance and radiation resistance). Moreover, due to the recent demand for energy savings, sliding materials are also required to be light in weight so that they can be driven by small loading.
Under the circumstances, silicon nitride or silicon carbide materials which are excellent in heat resistance and high in strength have hitherto been used as sliding materials, but these have a great kinetic coefficient of friction of 0.5-1.0 and are apt to cause wear of other materials and are not necessarily suitable as sliding materials. In addition, they are high in density and consume great energy for driving and have a large kinetic coefficient of friction. Moreover, they are brittle per se, and are considerably brittle if flawed and, moreover, have insufficient strength against thermal and mechanical shocks.
A s a means for solving these defects, ceramic composite materials (CMC) comprising composites of continuous ceramic fibers have been developed and used as sliding materials. These materials are high in strength a and tenacity even at high temperatures and have excellent impact resistance and environmental resistance, and, thus, they are being studied mostly in Europe and America as main refractory sliding materials.
On the other hand, as friction materials used in braking devices fitted to mass-transportation means such as large cars, ultra-high speed trains, and aircraft, carbon fibers-in-carbon often called C/C composites, which are very high in friction coefficient at high temperatures and light in weight, are widely used at present. In these mass-transportation means, it is common to continue braking for a long time depending on changes in driving circumstances or to repeatedly brake at high frequency. Therefore, in the case of braking devices using C/C composites as friction materials, the friction materials are exposed to high temperatures in the air for a long time. Accordingly, since friction materials using C/C composites are basically mainly composed of carbon fibers which readily burn at high temperatures, the carbon fibers react with oxygen under such conditions of being exposed to high temperatures for a long time to cause considerable wear or emission of smoke, leading to serious accidents. However, from the points of friction force at high temperatures and flexibility needed in fitting to disk brakes, substitutes therefor have not yet been discovered.
On the other hand, ceramic type SiCxe2x80x94C/C composite materials (CMC) comprising a composite of a ceramic matrix and fibers which are incorporated in the matrix have been developed in the following manner. That is, several hundred to several thousand ceramic long fibers of about 10 xcexcm in diameter are bundled to form a fiber bundle (yarn), these fiber bundles are arranged in planar or three-dimensional directions to form unidirectional sheets (UD sheets) or various cloths, or these sheets or cloths are laminated, thereby forming a preform (fiber preform) of a given shape, and a matrix is formed in the preform by a CVI method (chemical vapor impregnation method) or an inorganic polymer impregnating and firing method or by filling the preform with ceramic powders by a cast molding method and then firing the preform to form a matrix.
As examples of CMC, there have been known C/C composites comprising carbon fibers arranged in planar or a three-dimensional direction between which matrices comprising carbon are formed, and SiC fiber-reinforced Si-SiC composites formed by impregnating a molded body comprising SiC fibers and SiC particles with metallic silicon. Moreover, British Patent No. 1457757 discloses a composite material obtained by impregnating a C/C composite with metallic silicon to form SiC. In this composite material, a very common material is used as the C/C composite forming a skeletal part. That is, a phenolic resin as a binder is coated on carbon fibers of a suitable thickness, they are laminated in a uniform fiber direction so as to give the desired shape, and the laminate is put in a mold having a specific shape, followed by compressing and curing to obtain a molded body of C/C composite. This is fired and the fired body is impregnated with metallic silicon. The phenolic resin is carbonized by the firing, but the amount of the remaining carbon is at most about 50%, and after firing, many fine pores are present randomly around the carbon fibers. This is impregnated with metallic silicon, but it is very difficult owing to the random presence of the pores to uniformly impregnate the whole of the fired body with silicon. An SiC matrix is formed by the reaction of free carbon produced by carbonization of the phenolic resin used as a binder with metallic silicon used for impregnation, but, in this case, owing to the pores present randomly, the matrix does not become homogeneous and, simultaneously, metallic silicon also randomly reacts with carbon fibers to form an SiC layer on the carbon fibers. As a result, there is a problem in that the carbon fibers become short at the portion of the SiC layer being formed in at least a part of the composite material, resulting in deterioration of impact resistance, flexural strength, high lubrication and abrasion resistance.
Since C/C composites are high in tenacity, they are superior as braking members because of their excellent impact resistance, light weight and high hardness, but they are composed of carbon and hence cannot be used at high temperatures in the presence of oxygen and have a limit in the use as refractory sliding materials. Moreover, since they are relatively low in hardness and in compression strength, abrasion wear is large when used as sliding materials or braking members.
On the other hand, SiC fiber-reinforced Si-SiC composites are excellent in oxidation resistance and creep resistance, but are apt to be flawed on the fiber surface. Moreover, SiC fibers are high in wettability with Si-SiC and these are firmly bonded and, hence, drawing effect between the mother body and the fibers is small. Thus, they are inferior to C/C composites in tenacity, and therefore low in impact resistance and are not suitable for sliding materials having complicated shapes or having a thin wall portion, such as bearings and sliders. Furthermore, they lack reliability for materials usable for a long time as jigs for molten metal used at high temperatures. Thus, materials high in reliability and usable for a long period of time have not yet been provided.
That is, in the above-mentioned molten metal pump 14, the members which contact with molten metal 13 of high temperature of 500-800xc2x0 C., such as outer container 21, inner container 20, revolving shaft 17 and impeller 18, must be made of materials having impact resistance. Furthermore, if the material constituting the molten metal pump bleeds into the molten metal, quality of the plated articles is affected, and so there must be used materials which do not bleed out at high temperatures. Moreover, the materials constituting the molten metal pump must have oxidation resistance because they are sometimes used in the air. From these viewpoints, SIALON is used as the material of the outer container, etc. which contact with molten metal among the members constituting the molten metal pump.
An object of the present invention is to provide a novel SiCxe2x80x94C/C composite material which has a kinetic coefficient of friction within a range which does not damage sliding properties, has abrasion resistance, is light in weight, is excellent in impact resistance, creep resistance and spalling resistance, is high in hardness, and is hardly oxidized or worn even when exposed to high temperatures in the presence of oxygen. The material maintains the excellent impact resistance and light weight of C/C composites and is free from the defects of C/C composites which are used now as braking materials of mass-transportation means (i.e. changing operation at high frequency because of considerable wear which unavoidably occurs in the presence of oxygen caused by high temperatures). SIALON now used for the portions of molten metal pumps which require oxidation resistance has no problem in oxidation resistance, but is inferior in thermal impact resistance, and, for example, when it is used at 800xc2x0 C. for 100 hours, cracks occur at the portions in the vicinity of the surface of the molten metal.
The molten metal pump which constitutes one aspect of the present invention has been accomplished under the above circumstances, and thus another object of the present invention is to provide a molten metal pump from which the components do not dissolve out when used in molten metal and which has sufficient thermal impact resistance and oxidation resistance.
Still another object is to provide a novel SiCxe2x80x94C/C composite material which has high endurance usable as jigs for molten metal used at high temperatures of higher than 600xc2x0 C., preferably higher than 800xc2x0 C.
As a result of intensive research conducted by the inventors for attaining the above objects, it has been found that an SiCxe2x80x94C/C composite material comprising silicon carbide, carbon fibers and a carbon component other than carbon fibers and having a structure comprising a skeletal part and a matrix formed around the skeletal part, in which at least 50% of silicon carbide is of xcex2 type, the skeletal part is formed of carbon fibers and a carbon component other than carbon fibers, silicon carbide may be present in a part of the skeletal part, the matrix is formed of silicon carbide, the matrix and the skeletal part are integrally formed, and the composite material has a porosity of 0.5-5% and a two-peak type distribution of average pore diameter, can be used for making molten metal jigs, especially, molten metal pumps, thereby to be able to attain the above objects for the following reasons. Firstly, said SiCxe2x80x94C/C composite material is also excellent in oxidation resistance, creep resistance and spalling resistance and can also be used as sliding materials under such conditions that lubricants cannot be used due to high temperature conditions, in the presence of oxygen. Secondly, it exhibits excellent impact resistance and light weight as friction materials for brakes, shows sufficient abrasion resistance even in the presence of oxygen when used as friction materials for disk brakes which unavoidably generate high temperature, and does not require changing operation at high frequency as required for C/C composites and can be used continuously. Thirdly, it does not release components which contaminate the molten metal and has sufficient impact resistance and oxidation resistance. Thus, the present invention has been accomplished.
That is, the present invention provides an SiCxe2x80x94C/C composite material comprising silicon carbide, carbon fibers and a carbon component other than carbon fibers and having a structure comprising a skeletal part and a matrix formed around the skeletal part, characterized in that at least 50% of the silicon carbide is of xcex2 type, the skeletal part is formed of carbon fibers and a carbon component other than carbon fibers, silicon carbide may be present in a part of the skeletal part, the matrix is formed of silicon carbide, the matrix and the skeletal part are integrally formed, and the composite material has a porosity of 0.5-5% and a two-peak type distribution of average pore diameter.
Furthermore, the present invention provides a molten metal pump in which at least the portions which contact with molten metal are composed of the above SiCxe2x80x94C/C composite material. The molten metal pump according to another aspect of the present invention is preferably a molten metal pump which has a dross storage portion and a dross passage having openings at both ends and in which one end opening of the dross passage communicates with molten metal only at the surface portion of the molten metal outside the dross storage portion, another end opening of the dross passage communicates with molten metal only at the surface portion of the molten metal inside the dross storage portion, said dross passage is formed of a space defined by the inner wall of the outer container and the outer wall of the inner container constituting the dross storage portion, and the dross passage has an impeller on another end side which is fitted to a revolving shaft and brings about liquid flow from one end side and another end side, characterized in that at least the portions which contact with molten metal comprise the above SiCxe2x80x94C/C composite material. Furthermore, the molten metal pump is preferably one in which the portions which contact with the molten metal are the dross passage, the impeller and the revolving shaft. Moreover, the molten metal pump may be one which is used for molten zinc or molten aluminum.
The present invention further provides a method for producing the SiCxe2x80x94C/C composite material which comprises a step of keeping metallic silicon and a molded body comprising C/C composite or a C/C composite fired body in a furnace at a furnace inner temperature of 1100-1400xc2x0 C. and under a furnace inner pressure of 0.1-10 hPa for 1 hour or more with flowing an inert gas in an amount of 0.1 NL or more per 1 kg of total weight of the molded body or the fired body and the metallic silicon, thereby reacting the carbon component constituting the matrix of the C/C composite with the metallic silicon to form a matrix comprising silicon carbide, a step of raising the furnace inner temperature to 1450-2500xc2x0 C. with keeping the furnace inner pressure as it is, thereby melting and impregnating the metallic silicon into open pores of the molded body or fired body to grow silicon carbide and simultaneously sufficiently filling the remaining pores with the metallic silicon, and a step of increasing the furnace inner pressure to about 1 atm. with once reducing the furnace inner temperature to environmental temperature or keeping the furnace inner temperature as it is, and raising the furnace inner temperature to 2000-2800xc2x0 C., whereby the produced silicon carbide or metallic silicon filled in the pores is diffused from the matrix into the C/C composite composed of carbon fibers and a carbon component other than carbon fibers and is reacted with the carbon.
The SiCxe2x80x94C/C composite material of the present invention basically comprises 20-80% by weight of carbon and 80-20% by weight of silicon carbide, and the matrix comprising an SiC material is formed integrally between yarn assemblies comprising carbon fibers combined three-dimensionally and integrated so that they are not separated. Even if about 0.3% by weight of metallic silicon remains per total weight of the composite material, this gives substantially no influence to the performance of the composite material of the present invention. As mentioned hereinafter, when a layer of matrix formed of SiC material is provided, thickness thereof is preferably at least 0.01 mm, more preferably at least 0.05 mm, further preferably at least 0.1 mm.
Furthermore, in the novel SiCxe2x80x94C/C composite material of the present invention, it is preferred that the matrix has such a composition that content of silicon increases in inclined manner in proportion to the distance from the yarn. Moreover, the SiCxe2x80x94C/C composite material may contain at least one substance selected from the group consisting of boron nitride, boron, copper, bismuth, titanium, chromium, tungsten and molybdenum. In addition, it is preferred that the SiCxe2x80x94C/C composite material has a kinetic coefficient of friction of 0.05-0.6 at room temperature and a porosity of 0.5-5%.